<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .c1,.c3{
            background-color: red;
            height: 500px;
            width: 200px;

        }
        .c2{
            background-color: green;
            height: 500px;
            width: 200px;
        }

        #back_top{
            height: 40px;
            width: 80px;

            position: fixed;
            right: 40px;
            bottom: 40px;
            background-color: black;

            text-align: center;
            line-height: 40px;
        }
        #back_top a{
            color:white;
            text-decoration: none;
        }
        .hide{
            display: none;
        }
    </style>
</head>
<body>
<a name="xxx">这是顶部位置</a>

<div class="c1"></div>
<div class="c2"></div>
<div class="c3"></div>

<span id="back_top" class="hide">
    <a href="#xxx">回到顶部</a>
</span>



</body>
<script src="jquery.js"></script>
<script>
    $(window).scroll(function () {
        if ($(window).scrollTop() >= 200){
            $('#back_top').removeClass('hide');
        }else {
            $('#back_top').addClass('hide');
        }
    })
</script>
</html>